UPDATED 12:30 p.m. ET: This post has been updated to include the blizzard warning issued for Washington, D.C., and Baltimore, as well as new forecast information suggesting lower snow totals in New York City.

About 50 million people from the Appalachians to New York City are facing the likelihood of a crippling, life-threatening blizzard beginning on Friday and lasting through early Sunday. This would be the biggest winter storm of the season so far (named "Jonas" by The Weather Channel), providing a rude awakening to a region that saw a record warm December and hardly any snow at all so far this winter.

In Washington, where a dusting of an inch of snow ground Thursday evening's rush hour to a halt, the prospect of 12 to 24 inches of snow, along with strong winds, is particularly ominous.

Washington, D.C., forecast

This storm has the potential to do what no storm since 1922 has done in Washington: dump 20 or more inches of snow on the nation's capital, starting Friday afternoon. That is not guaranteed, and some computer models are hinting at a dreaded occurrence for D.C. snow lovers: a mix with sleet or rain for a time during the height of the storm. This would cut down on snowfall accumulations. However, the potential for a historic storm in Washington is there.

Top 3-day snowfall totals in the Washington, D.C., area.

Image: NWS/Jessica Plautz

In its blizzard warning statement, the National Weather Service (NWS) forecast office in Baltimore-Washington warned that travel in the region will become downright impossible on Friday night and Saturday, when the heaviest snows and strongest winds hit.

During the height of the storm, winds in Washington could reach 55 miles per hour, while coastal locations could even see higher gusts close to hurricane force, which is 74 miles per hour.

Here is the NWS's language concerning snowfall amounts in and around the city, using its ALL CAPS style for technical reasons: :

"ACCUMULATIONS...SNOW ACCUMULATION OF 18 TO 24 INCHES IN THE EASTERN SUBURBS OF WASHINGTON DC...AND 24 TO 30 INCHES IN THE WESTERN SUBURBS. THE CITY OF WASHINGTON DC IS EXPECTED TO RECEIVE AROUND 24 INCHES."

The NWS continued:

HEAVY SNOW AND BLOWING SNOW WILL CAUSE DANGEROUS CONDITIONS AND WILL BE A THREAT TO LIFE AND PROPERTY. TRAVEL IS EXPECTED TO BE SEVERELY LIMITED IF NOT IMPOSSIBLE DURING THE HEIGHT OF THE STORM FRIDAY NIGHT AND SATURDAY. VISIBILITY WILL BE REDUCED TO NEAR ZERO AT TIMES IN WHITEOUT CONDITIONS.

The weather agency warned people to plan ahead, adjust travel plans, and make plans for the elderly and those most at risk from the storm.

I will declare a snow emergency effective tomorrow morning at 9:30 am. Residents will need to move their cars from snow routes before then.

Overnight computer model runs have hinted that the heaviest snow may fall in higher elevations about 100 to 150 miles west and southwest of the city, since snowfall rates will be enhanced by the phenomenon of orographic lifting. This occurs when air is forced to rise. As it does so, it cools and moisture condenses into clouds and precipitation.

The mayor of Washington, Muriel Bowser, told reporters on Thursday morning that the city has not faced a storm like this before in modern memory.

"We're preparing for a blizzard. I've lived in D.C. most of my life and i don't know that I have lived through a forecast like this. It is an extremely large storm and will last for 36 hours. Residents and commuters should get home as early as possible on Friday."

Philadelphia & New Jersey

Snowfall amounts look to be lower — but still in blockbuster territory — in the Philadelphia area, which will be hit after D.C., on Friday night. The National Weather Service in Mt. Holly, New Jersey, is predicting a foot to a foot and a half for most of the metro area, including adjacent areas in New Jersey.

Here, too, snowfall rates and strong winds will make travel on Friday night and throughout Saturday extremely dangerous, if not impossible. The wind will be the result of the air pressure gradient between the rapidly intensifying low pressure area off the coast of Virginia and a strong high pressure area to the north. This air will be funneled, as strong winds are around skyscrapers, in between these pressure systems. The presence of the Gulf Stream current, with its deep, warm waters, will aid in the storm's rapid development.

Weather labels can't kill you, weather impacts can. We've been talking about this storm for five days. We've been talking about the impacts.

Sea surface temperature departures from average across the waters off the eastern seaboard.

Image: WeatherBell Analytics

Also, NWS forecasters in this area are watching out for an area of milder air that could turn snow to sleet and rain for a time, limiting accumulations in some areas. The reason for this warm air intrusion are twofold. First, the storm will be accompanied by a powerful low level jet stream at about 5,000 to 10,000 feet that will be screaming in from the east, moving over Atlantic Ocean waters that are considerably milder than average for this time of year. This jet should be sufficiently strong to change the snow to rain and sleet for a time near the New Jersey coast, but heavy precipitation falling to the west, near Philadelphia, could overcome this milder air through a separate process known as dynamical cooling.

This occurs when heavy precipitation falls and lowers air temperatures by decreasing air pressure and causing air to expand (the pressure gradient is stronger in the vertical air column than in the horizontal).

In a technical discussion on Thursday morning, the NWS noted that the big picture is one of increasing forecast confidence, but that smaller-scale details remain to be worked out. These details include where the heaviest snow bands — known as "mesoscale bands" due to their relatively small size — will set up, and where a mix or rain/snow line will establish itself. "IT IS THE MESOSCALE FEATURES ESPECIALLY THAT WILL HAVE AN IMPACT ON SNOWFALL INTENSITY AND AMOUNTS," the agency's forecasters wrote.

The NWS is expecting winds of up to 60 miles per hour to impact the New Jersey and Delaware coasts, which will pile water up against the shore during a period of high astronomical tides related to a full moon. However, in the Philadelphia metro area, winds may be below the threshold for a blizzard watch, so the NWS has held off on issuing one so far.

New York City

The NWS has issued a blizzard watch for New York City, Newark, N.J., and Long Island from Saturday morning through Sunday morning. This is more due to the combination of moderate to heavy snow coinciding with strong northeasterly winds, resulting in low visibility, than it does with actual snowfall amounts.

National Weather Service snowfall forecast in a high-end, but not worst-case, scenario.

Image: WeatherBell Analytics/NWS WPC

A blizzard has a technical, meteorological meaning, where the following conditions are expected to prevail for a period of three hours or longer: Sustained winds or frequent gusts to 35 miles an hour or greater; and considerable falling and/or blowing snow that reduce visibility frequently to less than ¼ mile.

Computer models continue to show that New York will be on the northern fridge of the heavy snowfall, with the potential for 8 to 12 inches of snow in the city. Areas to the northwest of the city are expected to see far less snow, while areas to the southwest may see more.

Predicting the precise location of the gradient between a foot of snow and just a few inches to nothing at all is one of the toughest forecast challenges involved in this storm, and for this reason, forecasts may change right on through the day on Saturday as snow and sleet fall on the Big Apple.

It's conceivable that 5 inches will fall in the Bronx, while Brooklyn, which is only about 8 miles southeast of there, will pick up a foot of snow out of this storm.

The NWS is being forthright about the high amount of uncertainty involved in the New York snow forecast:

WE ALSO WANT TO EMPHASIZE THE UNCERTAINTY WITH THE SNOWFALL FORECAST FOR THIS EVENT. ALTHOUGH THE MODELS HAVE BEEN HONING IN ON THIS WINTER STORM THREAT FOR REMARKABLY OVER A WEEK, THE SMALL SCALE DETAILS OF THIS STORM (THAT WILL DETERMINE EXACTLY WHERE THE HEAVIEST SNOW FALLS AND HOW FAR INLAND THE MIXING REACHES) IS YET TO BE DETERMINED. THEREFORE, THE GREATEST UNCERTAINTY WITH THE SNOWFALL AMOUNTS RESIDE ON THE NORTHERN EDGE OF PRECIP SHIELD (FROM I-78 NORTHWARD) AND IN THE COASTAL PLAIN DUE TO POTENTIALMIXING.

While New York is likely to see significant impacts from the storm, it is not likely to rank among the city's biggest snowstorms of all time.

Coastal Flooding

A less publicized, but extremely consequential, aspect of this storm will be the coastal flooding it may cause along the Delaware, Maryland and most especially, New Jersey shore. The combination of strong onshore winds, high waves, the storm's slow movement and high astronomical tides will result in moderate to possibly major flooding in Atlantic City and Cape May, New Jersey. Coastal flood watches are up for these areas, with the biggest concern occurring during the Saturday afternoon and Sunday morning high tides.

However, Hurricane Sandy-level flooding, which was mentioned as a possibility yesterday, is no longer looking likely. Partly, this is due to changes in the storm's timing, with peak winds and waves not occurring during the Saturday morning high tide, when water levels are naturally higher due to the full moon.

The NWS is warning that coastal flooding could be of long duration, lasting for three straight high tide cycles from Saturday into Sunday. Wave heights off the New Jersey and Delaware coasts are expected to build to 15 to 20 feet over the weekend, and a storm surge of around 3 feet on top of natural high tide levels may occur.

The maximum tide level in Atlantic City is forecast to be between 7.5 to 8 feet high, which would be close to the 10th-highest level on record for that location. In Cape May, New Jersey, the forecast water level is even higher, at 8 to 8.5 feet Saturday evening and Sunday morning. This would rank on the top 10 list for that location too, but would not approach Hurricane Sandy-like levels.

Mashable
is a global, multi-platform media and entertainment company. Powered by its own proprietary technology, Mashable is the go-to source for tech, digital culture and entertainment content for its dedicated and influential audience around the globe.